Electron-beam cured foams have been used as cushioning materials in automobile interiors, and a heat-resisting and heat-insulating materials.
Japanese Patent Laying-Open No. 265935/1988 discloses a cured foam consisting of an olefin resin comprising linear very-low-density polyethylene of more than 10 wt. %. The linear very-low-density polyethylene is made by copolymerizing ethylene with an .alpha.-olefin having more than 4 carbons. According to the disclosure, the olefin resin may be a polymer made from olefin monomers, such as polypropylene, or a copolymer mixture of ethylene/ethylacrylate. The linear very-low-density polyethylene may be composed by a process in which ethylene is copolymerized with an .alpha.-olefin having more than 4 carbons, such as butene-1, hexene-1, octene-1, 4-methylpentene-1, in order to add an appropriate number of short branches to the main linear chains, whereby the density is decreased.
The above document further discloses the following example. Ethylene/propylene random copolymer of 35 parts by weight having density of 0.89 g/cm.sup.3, Melt Index of 7.0 g/10 min, and melting point of 145.degree. C.; high-density polyethylene of 30 parts by weight having density of 0.957 g/cm.sup.3 and MI of 6.5 g/10 min; linear very-low-density ethylene/4-methylpentene-1 copolymer of 35 parts by weight having density of 0.905 g/cm.sup.3 and MI of 0.0 g/10 min; azodicarbonamide, as a foaming agent, of 15 parts by weight; trimethylpropane-trimethacrylate of 3.0 parts by weight; and a heat stabilizer of 0.5 parts by weight were mixed. The mixture was formed into a sheet of 1.5 mm thickness by an extruder. In order to cross-link the sheet, both surfaces were irradiated by an electron beam of 2.0 Mrad in an electron-beam irradiation apparatus. The sheet was then uniformly transported through a hot air stove at 250.degree. C. for heat foaming, whereby a sheet-type foam of about 4 mm in thickness was obtained.
The obtained cross-linked foam is of inferior tensile strength and dimensional stability, although it has good flexibility. That is, a foam produced in direct accordance with the technology disclosed in Japanese Patent Laying-Open No. 265935/1988 results in insufficient tensile strength and insufficient dimensional stability, due to inappropriateness in the selection of resins and the mixing ratio.
Additionally, Japanese Patent Laying-Open No. 155232/1981 and Japanese Patent Laying-Open No. 157839/1983 disclose a resin made of a copolymer of ethylene of which melting point ranges from 117.degree. C. to 123.degree. C. and density ranges from 0.890 g/cm.sup.3 to 0.910 g/cm.sup.3, and an .alpha.-olefin having 4 to 8 carbons. However, this resin is not comprised of 3 components, as in the above example from the prior art.